1. Field of the Invention
This invention relates generally to a current monitoring and control circuit and, more particularly, to a current monitoring and control circuit that employs a magnetic amplifier and a feedback circuit, where the amplifier circuit provides a bias current that causes the feedback circuit to operate the amplifier at a predetermined point to improve output linearity.
2. Discussion of the Related Art
Saturable core reactors have been employed in the art as variable impedance devices to detect direct current flowing in an operating circuit, while maintaining isolation between the operating circuit and an output circuit. A saturable reactor is a magnetic circuit element including a coil of wire wound around a magnetic core. The magnetic core significantly alters the behavior of the coil by increasing its magnetic flux and by confining most of the flux to the core. Magnetic flux density (B) is a function of applied magnet motive force (MMF), which is proportional to the ampere turns in the coil. The core includes a plurality of tiny magnetic domains made up of magnetic dipoles. These domains define the magnetic flux that adds to or subtracts from the flux provided by the magnetizing current. After overcoming initial friction, the magnetic domains rotate like small DC motors to become aligned with the applied field. As the MMF is increased, the domains rotate one by one until they all are in alignment and the core is saturated. Eddy currents are induced as the flux changes, causing added loss.
Magnetic amplifiers that employ saturable absorbers are known in the art, and are used for various applications, including current monitors for monitoring battery drain in spacecraft telemetry systems. Further, magnetic amplifiers are employed for current control in various systems, such as battery charging circuits and motor control circuits.
A conventional magnetic amplifier typically includes two saturable reactors having matched magnetic (permalloy) cores, each being wound with several turns of wires, such as 1500 turns of 38 awg wire. In a magnetic amplifier, each reactor winding is an amplifier gate winding. The two reactor gate windings are coupled in series and have opposing phase, i.e., are wound in opposite directions. The two reactors are positioned side by side and a bias winding, typically about 1000 turns (36 awg or larger), is wound around the reactors. A single control winding extends through the center of the cores, although, multiple turns may be used to increase amplifier sensitivity. The gate windings are coupled in series in opposing phase so that one reactor is reset as the other reactor drives towards saturation. A control current running through the control winding is measured by the magnetic amplifier by magnetic coupling.
An alternating current (AC) is applied to the gate windings and the output of the gate windings is full-wave rectified, filtered and resistively loaded to give a DC output voltage proportional to the control current. If the magnetic amplifier operated perfectly, and no current flowed through the control winding, then the current in the gate windings would oppose each other and the DC output voltage would be zero. The control current in the control winding moves the zero axis of the MMF produced by the gate current in the gate windings, and thereby reduces the inductance of the gate windings creating an imbalance between them. The greater the control current in the control winding, the smaller the inductance within a given range of current values. Thus, the greater the control current, the greater the imbalance between the gate windings, and the larger the output voltage.
In some applications, the bias winding is shorted or left open, and thus does not affect the magnetic coupling between the control windings and the gate windings. Sometimes it is desirable to shift the zero point of the output voltage when no control current is flowing through the control winding. By applying a bias voltage to the bias winding, the zero point of the output voltage is moved. This has application for determining the direction of the current through the control winding, as will be discussed in more detail below.
A magnetic amplifier operates similarly in principle to a current transformer. The ideal current transfer in the amplifier is expressed by:NcIc=NgIg+NbIbNc is the number of turns of the control winding, Ic is the control winding current, Ng is the number of turns of the gate windings, Ig is the gate winding current, Nb is the number of turns of the bias winding, and Ib is the bias winding current. The output voltage is Vo=IoRo when it is applied across a fixed load resistor, where:Io=(NcIc−NbIb)/Ng, and Ic=(NgIo+NbIb)/Nc
FIG. 1 is a schematic diagram of a conventional magnetic amplifier 10 of the type discussed above. The amplifier 10 includes a control winding 12, a bias winding 14, a first gate winding 16 and a second gate winding 18 coupled in series and opposing phase with the gate winding 16. In this design, the bias winding 14 is shorted and is not used. The amplifier 10 further includes an oscillator driver 24 that drives a transformer 26 with a suitable AC signal. The transformer 26 increases the voltage of the AC signal from the oscillator driver 24. The secondary winding of the transformer 26 is electrically coupled to the gate windings 16 and 18 and a rectifier 28 including a diode bridge. The AC signal applied to the gate windings 16 and 18 generates the gate winding current Ig. The gate winding current Ig is filtered and averaged by a filter 30 including a resistor 32 and a capacitor 34. Thus, the gate current Ig is rectified, filtered and resistively loaded to provide a DC output voltage Vo representative of the gate winding current Ic that is proportional to the control current Ic.
When there is no control current Ic in the control winding 12, the gate winding current Ig is nearly zero because of the equal and opposite windings of the gate windings 16 and 18 are equal and opposite. The control current Ic to be measured is applied to the control winding 12 and alters the gate winding current Ig in the gate windings 16 and 18 by magnetic coupling, as discussed above. Therefore, as the control current Ic increases either in the positive or negative direction, the output voltage V0 across the resistor 32 increases.
Because the gate windings 16 and 18 are driven by a square wave AC signal from the driver 24 and the output voltage Vo is full-wave rectified, the amplifier 10 cannot determine the direction of flow of the control current Ic. In other words, a positive or negative control current Ic in the control winding 12 generates the same positive DC output voltage V0. FIG. 2 shows a typical (ideal) control current Ic to output voltage transfer function for a current transducer or magnetic amplifier having an 80 amp operating range. The graph shows the output voltage V0 in relation to the control current Ic on the control winding 12, where the control current Ic changes linearly between −80 amps and +80 amps. However, the output voltage V0 goes from +5 volts to 0 volts, and then back to +5 volts, thus showing that the output voltage V0 does not identify the polarity of the control current Ic.
Further, the output voltage V0 of the amplifier 10 is not linear with respect to the control current Ic applied to the control winding 12. In other words, changes in the control current Ic are not reflected in changes in the output voltage V0 in a linear matter. The output linearity is affected by core mismatches and variations in the core construction and gate windings. Also, the effects of magnetizing current non-linearities in the B-H loop winding resistance and winding inductance can introduce errors over the full-scale output.
Also, the amplifier 10 is unable to measure a control current Ic on the control winding 12 below the gate winding's magnetizing current. Particularly, even if the control current in the Ic control winding 12 is zero, leakage in the gate windings 16 and 18 provide a current through the resistor 32 that provides an output voltage V0. Therefore, a control current Ic below the magnetizing current of the gate windings 16 and 18 cannot be measured because of system noise.
The bias winding 14 responds in a similar manner to the control winding 12 as the gate windings 16 and 18 through magnetic coupling. Because the bias winding 14 has more turns than the control winding 12 (generally 1000:1), a small amount of bias current Ib in the bias winding 14 would produce the same result as a much greater amount of control current Ic in the control winding 12. The bias winding 14 is generally used to shift the zero current operating point of the amplifier 10 to allow for discrimination of the control current Ic direction. In other words, the output voltage V0 based on the gate winding current Ig will be some value when a bias applied to the bias winding 14, but no control current Ic is flowing through the control winding 12. This discrimination is depicted in FIG. 3 showing the output relationship of a typical 80 amp magnetic amplifier having a 20 amp offset bias on the bias winding 14. By applying the bias current Ib to the bias winding 14, the new zero current operating point of the control winding 12 generates a 1.25 output voltage V0. Thus, the bias winding 14 controls the operating point of the amplifier 10. It is known in the art to provide dual magnetic amplifiers, one including a bias voltage on the bias winding 14, to provide an indication of the current direction through the control winding 12.